Evidence from the Motions of Old Stars that the Galaxy Collapsed

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1 Evidence from the Motions of Old Stars that the Galaxy Collapsed O. J. Eggen, D. Lynden-Bell, and A. R. Sandage 1962, ApJ, No. 136, p. 748 Presented by: Michael Solway All equations and figures from here on are taken from this paper.

2 Main Findings Observations of the velocities and ultraviolet excesses δ(u B) of 221 dwarf stars in the solar neighborhood suggest that: Stars with large excess (low metal abundance) Move in highly elliptical orbits Have small angular momenta Formed at any height above the galactic plane Stars with little or no excess Move in nearly circular orbits Have large angular momenta Formed near the galactic plane therefore The oldest stars formed before the galaxy collapsed to a thin disk in equilibrium Young stars formed after the collapse

3 2D Orbits in Dynamical Equilibrium Cylindrical symmetry (R, φ, Z) coordinates (U, V, W) velocities Energy and angular momentum equations: or Isochrone potential: Boundary conditions: M = total mass of galaxy = M b = constant length = 2.74 kpc calculated from R = 10 kpc and V = 250 km/sec

4 Circular Velocity

5 Shapes of Orbits Fig. 2 Fig. 3

6 Assumptions to Simplify a Contracting Potential Neglecting the Z-motion While a star traverses one Z-semiperiod, it moves very little in R The potential is separable Adiabatic invariant: Ż dz = const The potential is axially symmetric at all times The galaxy never had a bar Angular momentum is conserved for each element No h exchange through pressure gradients or magnetism No accretion of matter onto a star s surface Any turbulence is much smaller than the size of the galaxy Contracting Isochrone potential b changes with time

7 Change of the Eccentricity Case 1: potentials change little during one galactic rotation Adiabatic invariant: P R dr = const Invariant eccentricity e*: e* = e for h 2 >> 4bGM e* and e don t differ by more than 0.1: e is an adiabatic invariant Case 2: potentials change rapidly Mass concentration increases in a time shorter than the galactic period On average, e increases e decreases for stars at their perigalacticum W(Z = 0) either increases or remains the same Z max decreases or stays the same

8 Data of 221 Stars (U, V, W)-velocity vectors and photometry measured 108 from (Eggen 1961): 4000 stars with accidental error in U < 1.5 km/sec 113 from (Eggen 1962): stars with velocity > 100 km/sec Uncertainties in the (U, V, W) vectors < ± 20 km/sec All the stars are within a few hundred parsecs of the sun All are believed to be dwarfs From spectroscopic or trigonometric parallax data Distances measured by force-fitting to the Hyades main sequence (Sandage and Eggen 1959) Some bias against stars with nearly circular orbits No bias in the ultraviolet excesses Some neglect of stars with intermediate velocities

9 Correlation of e and δ(u B)

10 Correlation of W and δ(u B) Z max differs by at least a factor of 25 between stars with the biggest and smallest ultraviolet excesses

11 Correlation of h and δ(u B) h and δ(u B) are independent of time Corresponds to the correlation between e and δ(u B)

12 h vs. R in Dynamical Equilibrium Material with the same mean h as that of the oldest stars with the greatest-velocity (h = kpc km/sec) is in circular orbits at distances < 5 kpc from the galactic center at present

13 Interpretation The galaxy collapsed onto a disk either after or during the formation of the oldest stars The collapse was very rapid: ~ 10 8 years Slow collapse cannot explain the presence of high eccentricities Why are the oldest stars now moving in nearly rectilinear orbits and the youngest in nearly circular orbits? The galaxy did not settle to equilibrium before the first stars formed If the early galaxy was in dynamical equilibrium, then the oldest stars would have formed at < 5 kpc from the center 1 st generation stars formed during the collapse and now have high e 2 nd generation stars formed from the collapsed gas and now have nearly circular orbits Present galaxy < 1 / 10 radial scale of the initial protogalaxy

14 Correlation of h and Rmax

15 Correlation of h and e Why are there no stars with high e and high h? More likely to find stars whose apogalactica are near the sun Stars with small apogalactica are more favored by the stellar density gradient Gas density at large distances was insufficient to form stars before equilibrium was reached

16 How the Galaxy Formed Intergalactic material began to collapse about years ago As it was collapsing, condensations formed and later became globular clusters and globular cluster-like stars 1 st generation stars e increased during the rapid collapse A thin disk formed The collapse stopped in the radial direction due to rotation Nothing could stop it in the Z direction 1 st generation stars enriched the remaining gas with metals Later generation stars show a small ultraviolet excess The gas separated from the stars at their perigalactica, settled into circular orbits, and continued to produce stars 2 nd generation stars initially had circular orbits Dynamical equilibrium was achieved and e stopped changing